umount() and umount2() remove the attachment of the (topmost)
filesystem mounted on target.
Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is
required to unmount filesystems.
Linux 2.1.116 added the umount2() system call, which, like umount(),
unmounts a target, but allows additional flags controlling the
behavior of the operation:
MNT_FORCE (since Linux 2.1.116)
Ask the filesystem to abort pending requests before attempting
the unmount. This may allow the unmount to complete without
waiting for an inaccessible server, but could cause data loss.
If, after aborting requests, some processes still have active
references to the filesystem, the unmount will still fail. As
at Linux 4.12, MNT_FORCE is supported only on the following
filesystems: 9p (since Linux 2.6.16), ceph (since Linux
2.6.34), cifs (since Linux 2.6.12), fuse (since Linux 2.6.16),
lustre (since Linux 3.11), and NFS (since Linux 2.1.116).
MNT_DETACH (since Linux 2.4.11)
Perform a lazy unmount: make the mount point unavailable for
new accesses, immediately disconnect the filesystem and all
filesystems mounted below it from each other and from the
mount table, and actually perform the unmount when the mount
point ceases to be busy.
MNT_EXPIRE (since Linux 2.6.8)
Mark the mount point as expired. If a mount point is not
currently in use, then an initial call to umount2() with this
flag fails with the error EAGAIN, but marks the mount point as
expired. The mount point remains expired as long as it isn't
accessed by any process. A second umount2() call specifying
MNT_EXPIRE unmounts an expired mount point. This flag cannot
be specified with either MNT_FORCE or MNT_DETACH.
UMOUNT_NOFOLLOW (since Linux 2.6.34)
Don't dereference target if it is a symbolic link. This flag
allows security problems to be avoided in set-user-ID-root
programs that allow unprivileged users to unmount filesystems.

The error values given below result from filesystem type independent
errors. Each filesystem type may have its own special errors and its
own special behavior. See the Linux kernel source code for details.
EAGAIN A call to umount2() specifying MNT_EXPIRE successfully marked
an unbusy filesystem as expired.
EBUSY target could not be unmounted because it is busy.
EFAULT target points outside the user address space.
EINVAL target is not a mount point.
EINVAL umount2() was called with MNT_EXPIRE and either MNT_DETACH or
MNT_FORCE.
EINVAL (since Linux 2.6.34)
umount2() was called with an invalid flag value in flags.
ENAMETOOLONG
A pathname was longer than MAXPATHLEN.
ENOENT A pathname was empty or had a nonexistent component.
ENOMEM The kernel could not allocate a free page to copy filenames or
data into.
EPERM The caller does not have the required privileges.

umount() and shared mount points
Shared mount points cause any mount activity on a mount point,
including umount() operations, to be forwarded to every shared mount
point in the peer group and every slave mount of that peer group.
This means that umount() of any peer in a set of shared mounts will
cause all of its peers to be unmounted and all of their slaves to be
unmounted as well.
This propagation of unmount activity can be particularly surprising
on systems where every mount point is shared by default. On such
systems, recursively bind mounting the root directory of the
filesystem onto a subdirectory and then later unmounting that
subdirectory with MNT_DETACH will cause every mount in the mount
namespace to be lazily unmounted.
To ensure umount() does not propagate in this fashion, the mount
point may be remounted using a mount() call with a mount_flags
argument that includes both MS_REC and MS_PRIVATE prior to umount()
being called.
Historical details
The original umount() function was called as umount(device) and would
return ENOTBLK when called with something other than a block device.
In Linux 0.98p4, a call umount(dir) was added, in order to support
anonymous devices. In Linux 2.3.99-pre7, the call umount(device) was
removed, leaving only umount(dir) (since now devices can be mounted
in more than one place, so specifying the device does not suffice).

This page is part of release 4.13 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 UMOUNT(2)